The present invention relates to a switching power supply, and particularly to a forward converter for an off-line switching power supply.
Switching mode power supplies are often used in small-size electrical appliances such as a personal computer. A plurality of low voltage stress forward configurations of switching mode power supplies have been developed. However, such configurations still require a large input filter to reduce the noises caused by a pulsating input current of a forward converter.
U.S. Pat. No. 5,523,936 issued to the inventor of the present invention and Hwang, discloses a built-in input filter forward converter. A power switch and two cross-coupled capacitors are respectively connected to a split-winding transformer. When the power switch is turned off, an input current of the converter can flow through a path provided by the cross-coupled capacitors. However, such an arrangement requires a high voltage rating power switch in off-line applications, thus, the cost is increased and the reliability is reduced. Further, a resistance of the power switch is large and a power loss will also be increased.